In a manufacturing process of a NAND flash memory, a silicon nitride film is formed above memory cells and selection gates formed on a wafer. Trenches or holes for contact plugs that connect the selection gates to source lines or bit lines are formed by RIE (Reactive Ion Etching) using the silicon nitride film as a stopper film. Contact plugs are embedded in the formed trenches or holes with barrier metal interposed therebetween.
However, in the conventional manufacturing process, active hydrogen is generated in a silicon nitride film at a heat treatment step for a wafer after formation of the silicon nitride film and the generated active hydrogen reaches a tunnel oxide film, which deteriorates the tunnel oxide film.
Therefore, there has been a demand for suppression in deterioration of the memory cells due to active hydrogen and improvement in reliability of the memory cells.